This invention relates to directional beam forming antennas, and in particular to metallic grating type spatial filters for suppressing the sidelobes of beams transmitted by such antennas.
The performance of phased arrays and other directional beam forming antennas is often degraded by the presence of sidelobes and grating lobes in the transmitted beam. A particular problem is represented by the residual grating lobes that plague limited sector scanning and multiple beam arrays in airport precision-approach radar systems and synchronous satellite communications antennas. In the past, for each individual case, sidelobe problems have been overcome by redesigning the antenna. Such an approach is, of course, both inflexible and expensive. A substantial improvement on previous techniques dealing with this problem is disclosed in our co-pending patent application Ser. No. 678,516, filed Apr. 19, 1976, entitled "METALLIC GRATING SPATIAL FILTER FOR DIRECTIONAL BEAM FORMING ANTENNA". However, while the layered dielectric filter described therein avoids the need to redesign the antenna for each application, its weight and cost could be improved upon.
The present state-of-the-art also includes filters comprised of multiple parallel metallic gratings. Typical of this type of filter is the device shown in U.S. Pat. No. 2,763,860 entitled HERTZIAN OPTICS, issued to A. Ortusi et al, Sept. 18, 1950. Devices of this type, however, focus the transmitted beam and do not necessarily eliminate sidelobe and other unwanted portions of the beam in the manner desired.
The present invention is directed toward providing a spatial filter that retains the advantages of the layered dielectric filter without focusing the beam and at the same time significantly reducing cost and weight requirements.